Analysis of in situ formation of HAP
The levelling effect was considered when investigated the kinetic characteristics of Ca2+, thus 3.0 wt.%, 1.0 wt.% and 0.6 wt.% of calcium acetate were used to explore the effect of the formation of HAP on Ca2+ consumption. The relationships of Ca2+ consumption rate (Fig. 1a) and its reciprocal 1/ (Fig. 1b) over time of samples were plotted intuitively, and linear relations were fitted in Fig. 1b with y for 1/ and x for time. The fitted lines overlapped in 3.0 wt.% and 1.0 wt.% CA, therefore, Fig. 1b showed four linear formulas. The difference between formulas ① and ② indicated the treated sol consumed more Ca2+ with a faster rate in 0.6 wt.% CA, which was in agreement with expectation that not only the cross-linking reaction between NaAlg and Ca2+, but also the ion exchange reaction among Ca2+, PO43− and OH− to form HAP occurred. In addition, 0.81 g HAP was produced in 0.6 wt.% CA by calculation, and 0.89 g in 3.0 wt.% CA according to ICP in Table S1. Faster reaction rate and higher yield implied that 3.0 wt.% CA was the best adoption in subsequent experiments.
The in-situ generation of hexagonal HAP was monitored by XRD (Fig. 1c) and SEM (Fig. 1d-i). CaAlg and HAP began to form as soon as calcified, and the crystallinity of HAP might be confirmed after 10 min (Sinulingga et al. 2021). CaAlg exhibited a smooth layered microstructure (Fig. 1d), while the treated sol generated CaAlg and HAP after calcification for 1 min, with a dense arrangement of spherical nanoparticles of a diameter about 2 nm on the surface of CaAlg (Fig. 1e). Then the nanosheets were grown in the HAP spherical particles in all directions, forming hierarchical microspheres with one-dimensional nanowires and two-dimensional nanosheets, resulting in urchin-like microspheres with average diameters of 3 µm, 4 µm and 6 µm, when calcified for 5 min, 10 min and 30 min (Fig. 1f-h), respectively. However, when calcified for more than 30 min, the HAP microspheres narrowed their size and tended to be wrinkled sponge microspheres with an average diameter of 2 µm after 90 min (Fig. 1i), which was consistent with the constant after 90 min in Fig. 1a. According to Ostwald's "rule of stages", this may be in the reason that HAP tended to form unstable and more soluble phases in the early stage of reaction, and would transform into more stable phases in the later stage (Daryan et al. 2020). CaAlg/HAP semi-interpenetrating polymer network may more tightly contact for wrinkled sponge microspheres with a larger superficial area than smooth microspheres, which was beneficial for HAP to adhere to the layered structure of CaAlg (Fong et al. 2017).
Flame retardant properties of CaAlg and CaAlg/HAP
Thermal stability
TGA was conducted under N2 to explore the pyrolysis process without oxidation reaction, and the two samples showed a similar tend with three intense weight loss processes mainly displayed in Fig. 2a-b. The first stage of the weight loss below 200 ℃ was mostly the removal of free water and bound water in the samples. The first similar weight loss process suggested that the content of water and the combination with water in both samples were alike. The second stage (200–300 ℃) and third stage (300–400 ℃) were the primary and complex pyrolysis processes, including the breaking of glycosidic bond, dehydration, decarboxylation, decarbonylation, etc.(Zhang et al. 2021). According to Table. S2, CaAlg/HAP proceeded the second stage of pyrolysis at the lower temperature and occurred less weight loss after 200 ℃, resulting in its residue of 11.58 %, which was more than that of CaAlg in 900 ℃. Possibly, HAP promoted the formation of more stable char and less flammable small molecules during the thermal decomposition in N2, granting the CaAlg better flame retardancy.
As shown in Table. S1, HAP accounted for 0.974 wt.% in CaAlg/HAP, and HAP was assumed no mass change in TGA for it can exist stably below 1000°C (György et al. 2019). If HAP had no effect on the thermal decomposition of CaAlg, the weight loss of CaAlg doped HAP calculated by the theoretical formula should be the same as the experimental CaAlg/HAP. Theoretical residue was calculated according to the following formula (Zhang et al. 2019):
, (1)
where mcal, mCaAlg and mHAP represented the theoretical char of CaAlg/HAP, experimental char of CaAlg, and char of HAP (calculated as 100%), respectively, and wtHAP was the mass proportion of HAP in CaAlg/HAP. The obtained theoretical TG curve (blue curve in Fig. 2a) was just a little bit above that of CaAlg. It showed that HAP promoted the carbonization of CaAlg during thermal degradation, possibly because of the thermal insulation of HAP (Liu et al. 2018).
The pyrolysis of samples in air was more complex - oxidation reactions occurred between O2 and the products of CaAlg, leading to a further loss of weight (Xu et al. 2019). As seen in Fig. 2, CaAlg before 402 ℃ and CaAlg/HAP before 384 ℃ in both atmospheres were in consistence, however, subsequently, the oxygen reacted with the combustible gas born from previous stage, causing one more obvious pyrolysis in Fig. 2c-d. As shown in Table. S2, the lower Rmax and more residue of CaAlg/HAP in air indicated that HAP can prevent oxygen from participating in the thermal decomposition of alginate at high temperature, resulting in a markedly better thermal stability of CaAlg/HAP, compared with that of CaAlg.
SEM and XRD of CaAlg and CaAlg/HAP after calcination in air
According to TGA, CaAlg and CaAlg/HAP were calcined in air at 250 ℃, 450 ℃ and 750 ℃ for 1 h to explore the morphology and composition changes of the prepared samples. Figure 3a-c show that the morphology of CaAlg was slightly irregular deformation when heated at 250 ℃ and 450 ℃, while at 750 ℃, a densely arranged residue structure appeared. However, CaAlg/HAP was significantly different from CaAlg. When heated at 250 ℃, the diameter of HAP microparticles started narrowing to about 1–2µm, and retained the wrinkled sponge appearance, as shown in Fig. 3d, while it turned to less than 1 µm at 450 ℃, there were many nanopores on the compact surface of the residue which can be observed in Fig. 3e. The XRD in Fig. 3h meant that these nanopores were formed due to the generation and discharge of CO2 from the decomposition of CaCO3. As seen in Fig. 3f, when CaAlg/HAP was heated at 750 ℃, the HAP particles continued shrinking to the diameter of 20 nm and the nanoparticles were closely arranged on the surface of CaAlg, indicating that the compact surface structure prevented the thermal degradation of CaAlg (György et al. 2019).
As observed in Fig. 3g, there was no CaCO3 formed in CaAlg until about 450 ℃, and a little CaCO3 decomposed into CO2 and Ca(OH)2 at 750 ℃. While as seen in Fig. 3h, the generation of CaCO3 began in CaAlg/HAP when heated at 250 ℃, and possibly the inner layer of alginate was covered and protected by CaCO3. At 450 ℃, CaCO3 cracked to produce CaO and H2O, and when the temperature rose up to 750 ℃, the crystallinity of CaO increased, indicating that more CaO formed. It showed that HAP can effectively promote the carbonization of CaAlg during combustion, while HAP crystal not only was undecomposed but also a higher crystallinity at higher temperature emerged.
In terms of the above, HAP with ultra-high temperature resistance absorbed heat to form smaller particles that were tightly packed onto the surface of CaAlg. What’s more, HAP promoted faster carbonization of CaAlg and produced stable CaCO3 at around 250 ℃, and CaO at around 450 ℃ or even higher temperature, which prevented further pyrolysis, and thus improved the flame retardant performance of the hybrid material.
Combustion properties
Table 1
Data from LOI, UL-94, CONE, and MCC.
Samples
|
LOI (%)
|
UL-94
|
CONE
|
MCC
|
TTI (s)
|
TSR (m2/m2)
|
EHC (MJ/kg)
|
Residue(%)
|
PHRR1 (W/g)
|
PHRR2 (W/g)
|
THR (kJ/g)
|
Residue(%)
|
CaAlg
|
46.5±0.5
|
V-0
|
16.0± 1.0
|
3.6± 0.2
|
26.4± 0.1
|
50.7± 0.3
|
10.7± 0.2
|
9.8± 1.4
|
0.8± 0.1
|
42.8± 0.1
|
CaAlg/HAP
|
67.0± 1.0
|
V-0
|
16.0± 1.0
|
1.12± 0.3
|
12.3± 0.1
|
61.7± 0.2
|
9.5± 0.1
|
3.8± 1.4
|
0.6± 0.5
|
52.5± 0.4
|
TTI: time to ignition; EHC: effective heat combustion; PHRR1 and PHRR2: the first and second peak of HRR in MCC; THR: total heat release. |
As it can be seen from Table 1, the LOI of CaAlg/HAP was 67 %, which was 20.5 % higher than that of CaAlg (Liu et al. 2021), and CaAlg/HAP cannot be ignited in the UL-94 test. The EHC of CaAlg/HAP was reduced to half of CaAlg in the CONE test, PHRR and THR in MCC were significantly reduced, and the residue in CONE and MCC of the samples showed the same tendency with TGA, as displayed in Fig. 2. All these results indicated that the doping of HAP was beneficial for the improvement of the thermal stability and flame retardant properties of CaAlg.
Figure 4a shows the variation of the total smoke release (TSR) from the samples during the CONE test over time. The TSR of CaAlg/HAP was remarkably less than that of CaAlg, possibly because the more char the hybrid material produced, the less smoke containing toxic components was released. Importantly, the less smoke release can reduce casualties in fire (Kong et al. 2021; Yu et al. 2021a).
Since most of the oxygen may be consumed in the real fire cases, the pyrolysis of samples under an anaerobic condition can be studied via MCC. MCC simulates heat release throughout the decomposition of samples in nitrogen (Mensah et al. 2018), thus it may correspond with the TGA in N2 theoretically. As seen in Fig. 2b and Fig. 4b, the third weight loss stage in TGA at 300–400°C corresponded to the first heat release stage in MCC, while 400–600°C was the second heat release stage with little weight change, suggesting that the chemical energy was converted into heat at 300–600 ℃. The less weight loss and HRR of CaAlg/HAP implied that the pyrolysis of hybrid was effectively prevented. There were two potential reasons for the decrease of HRR (Liu et al. 2016b). One was that HAP may catalyze the formation of stable char from CaAlg and contributed to a reduction of combustible gas release, which was confirmed by the results of TGA (Fig. 2) and XRD (Fig. 3g-h). The other one was that HAP facilitated the formation of non-flammable gases from alginate such as CO2 and water, which would dilute the concentration of flammable products.
TG-FTIR
In order to investigate whether the improvement of flame retardant properties was related to the synergistic effect of both of the aforementioned reasons, TG-FTIR was conducted to explore the generation of CO2 and H2O during the whole pyrolysis process.
The main functional groups containing carbonyl group C = O at 1795 cm− 1, carbon monoxide CO at 2182 cm− 1, carbon dioxide CO2 at 2362 cm− 1, -CH at 2962 cm− 1, and = CH at 3018 cm− 1, and H2O at 3732 cm− 1 from the 3D images of TG-FTIR were selected to explore the pyrolysis of CaAlg (Fig. 5a) and CaAlg/HAP (Fig. 5b). It was found that C = O, CO and CO2 showed a similar generation trend, and all of them were produced apparently above 250 ℃, indicating that the pyrolysis of glycosidic bonds was started and a variety of complex reactions occured (Fig. 5c-e). What’s more, CaAlg/HAP generated more C = O, CO and CO2 than CaAlg did, especially over 350 ℃, reflecting that the carboxyl groups or similar groups in CaAlg/HAP were more reactive when heated, which activated more decarboxylation and decarbonylation. The -CH and = CH were markedly generated around 450 ℃, as shown in Fig. 5f-g, suggesting that the macromolecular residues from the samples went on a further crack into smaller chains. As seen in Fig. 2h, the water was formed throughout the pyrolysis process periodically. Not only -CH and = CH but also H2O were decreased in CaAlg/HAP, implying that its degree of pyrolysis was significantly reduced. The above functional groups of CaAlg/HAP exhibited a similar trend as CaAlg did, however, it produced more CO2 above 350 ℃ and the degree of pyrolysis in the later stage was alleviated. Therefore, it can be inferred that, for the carbonization mechanism of CaAlg/HAP, HAP promoted the decarboxylation of alginate, which may because the interface sites between HAP and alginate was conducive to the occurrence of decarboxylation (Fu and Mei 2021). Since HAP was not decomposed to produce phosphoric acid when heated, its flame-retardant mechanism is not similar to most of phosphates that generate phosphoric acid during heating (Sun et al. 2021a).
Collectively, it can be confirmed that the increasing release of CO2 diluted the combustible gas in the CaAlg/HAP combustion system, and turned to an isolation among air, fuel and heat. In general, it was the combination for production of more stable char earlier and generation of more CO2 over 350 ℃ together that improved the flame retardancy of the hybrid material. The high flame retardancy of CaAlg/HAP can be mainly attributed to the synergistic effects of gas phase and condensed phase.
Py-GC-MS
According to the curves in Fig. 6 and the data in Table S3-4, when the temperature rose from room temperature to 250 ℃, CaAlg/HAP produced less CO2 than the CaAlg, indicating that alginate was effectively prevent in the early stage of pyrolysis before 250 ℃. However, when the temperature rose to 450 ℃, CaAlg/HAP generated more CO2 than CaAlg, and as the temperature continued rising to 750 ℃, there was a further increase of CO2, which was in line with more generation of CO2 from 350 ℃ in Fig. 5d. The types and amount of pyrolysis products produced by CaAlg/HAP were less than CaAlg. Such as CaAlg/HAP produced less acetic acid than CaAlg significantly at 450 ℃, and it produced less kinds of products than CaAlg at 750 ℃. It was proved that the flame retardant mechanism of CaAlg/HAP was various at different stages of pyrolysis. The function of solid phase was the main flame retardant mechanism from room temperature to 350°C, while the synergistic effect of the solid phase and the gas phase worked at 350–750°C.
Flame-retardant mechanism
The speculative pyrolysis mechanism of CaAlg/HAP with no oxygen involved is presented in Scheme 2, which was extrapolated from the pyrolysis products listed in Table S3 (Xu et al. 2021). According to the morphology and XRD patterns of CaAlg/HAP calcined at 250 ℃, 450 ℃, and 750 ℃ (Fig. 2), micron-sized HAP particles with low energy were changed into nano-sized with high energy, because of gaining heat. Therefore, during the whole pyrolysis process, part of the heat was received by CaAlg/HAP to obtain a compact structure on the surface of CaAlg, which to some extent delayed the thermal decomposition.
As shown in Fig. S3, hydrogen bonds may be formed between HAP and M blocks, which improved the thermal stability of M blocks. The interface site between HAP and alginate activated the decarboxylation of “egg-box” structure related to G blocks, which resulted in the generation of CaCO3, CaO and CO2 to protect alginate when heated. Dehydration and slight decarboxylation occurred in alginate below 250°C, thus CO2 and H2O were the main products. Less CO2 was produced from CaAlg/HAP below 250°C, indicating that the formation of CaCO3 protected CaAlg and reduced pyrolysis. The glycosidic bonds inside CaAlg can be broken into M and G blocks above 250 ℃. M blocks was further cracked into butanedione, acetone and acetic acid in CaAlg as a general rule, while these products significantly reduced in CaAlg/HAP, indicating that less pyrolysis occurred in M blocks. However, although CO2 accounted for 92.65 % and other components were quietly rare at 750 ℃, a little bit of pyrolysis products from G blocks such as acetaldehyde, 2-methylfuran and furfural can be found from CaAlg/HAP. These results suggested that HAP protected M blocks by forming hydrogen bonds with it, and promoted the partial decomposition of the “egg-box” structure to produce CaCO3 and CaO to enhance the thickness and strength of the carbon layer as well. Moreover, CO2 from that was to dilute the flammable gas and prevented further decomposition of alginate at high temperature. The combustion process of CaAlg/HAP in air is exhibited in Fig. 7, and more CO2 was produced than in N2 due to the oxidation reactions.